Streamer uses KafkaW

Created by: michele-brambilla

This PR introduces two main changes

1. the streamernow uses KafkaW instead of librdkafka;
2. the connect() task is now executed in task-based parallelism instead of thread-based;

and a new feature

3. added tests for the Streamer.

Consequences:

1. makes the filewriter implementation more uniform. KafkaW is the only explicit connection with librdkafka;
2. let us remove explicit locks and condition variables. The flow is now more linear;
3. needs the fake librdkafka implementation to be specified using -DFAKE_RDKAFKA=<path> when invoking cmake.